1. Field of the Invention
The present invention relates to an extreme ultraviolet (EUV) light source apparatus to be used as a light source of exposure equipment.
2. Description of a Related Art
In recent years, as semiconductor processes become finer, photolithography has been making rapid progress toward finer fabrication. In the next generation, micro-fabrication at 70 nm to 45 nm, further, micro-fabrication at 32 nm and beyond will be required. Accordingly, in order to fulfill the requirement for micro-fabrication at 32 nm and beyond, for example, exposure equipment is expected to be developed by combining an EUV light source for generating EUV light having a wavelength of about 13 nm and reduced projection reflective optics.
As the EUV light source, there is an LPP (laser produced plasma) type light source using plasma generated by irradiating a target with a laser beam. The LPP type light source has advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made larger, that light emission of only the necessary waveband can be performed by selecting the target material, and that an extremely large collection solid angle can be ensured because it is a point light source having substantially isotropic angle distribution and there is no structure such as electrodes surrounding the light source. Therefore, the LPP type light source is predominant as a light source for photolithography.
FIG. 52 is a schematic diagram for explanation of a configuration and an alignment method of a conventional LPP type MTV light source apparatus. The LPP type EUV light source apparatus utilizes EUV light radiated from plasma generated by irradiating a target material, which is supplied into a vacuum chamber (EUV chamber), with a laser beam.
As shown in FIG. 52, the LPP type EUV light source apparatus includes a driver laser for generating a laser beam such as a short-pulse CO2 laser beam, a laser beam focusing optics (including a reflection mirror and an off-axis parabolic mirror, for example) for focusing the laser beam on the target material to turn the target material into plasma, and an EUV chamber in which EUV light is generated. An EUV collector mirror for collecting the EUV light radiated from plasma to output the EUV light is provided within the EUV chamber.
As the target material, for example, a metal such as tin (Sn) or lithium (Li) is used. The target material is melted in a target supply unit and pressurized by an inert gas such as argon (Ar), and a jet of the target material is injected from a target nozzle in which a microscopic hole is formed. When regular disturbance is provided to the jet of the target material by using a vibrator attached to the target nozzle, the jet of the target material is split into droplets having uniform diameters and intervals.
The laser beam generated from the driver laser is guided by the laser beam focusing optics and applied to the target via a window. The target irradiated with the laser beam is turned into plasma, and various wavelength components including the EUV light are radiated.
The EUV collector mirror is a spheroidal mirror having a first focal point and a second focal point, and placed such that the first focal point is located at a focusing point of the laser beam, which corresponds to a plasma emission point (EUV emission point), and the second focal point is located at an intermediate focusing point (IF), which corresponds to a point light source for an exposure unit. The reflection surface of the EUV collector mirror is coated with a multilayer-coating in which molybdenum (Mo) and silicon (Si) thin coatings are alternately stacked (Mo/Si multilayer-coating), for example, for selectively reflecting a desired wavelength component (e.g., a component having a wavelength of 13.5 nm). Thereby, the EUV collector mirror selectively reflects and collects the EUV light radiated from the plasma, converges the EUV light on the second focal point of the EUV collector mirror, and supplies the EUV light to a device such as an exposure unit utilizing EUV light.
Here, the driver laser beam and the EUV light are invisible light, and therefore, the location adjustment of the optics has been not easy. For example as shown in FIG. 52, when the EUV collector mirror is changed due to the heat of the laser beam, the location of the intermediate focusing point (IF) is shifted. Conventionally, an IF location sensor has been inserted between the EUV light source apparatus and the exposure unit, an image of the EUV emission point formed near the intermediate focusing point (IF) has been detected, and the location and the angle, at which the EUV collector mirror is placed, have been adjusted based on the detection result.
As a related technology, Japanese Patent Application Publication JP-P2007-109451A discloses an alignment method including the steps of placing an IF location sensor near an intermediate focusing point (IF) at fixed intervals, observing an image of an EUV emission point, and adjusting alignment of an EUV collector mirror based on the result, in an EUV light source apparatus.
However, according to the above-mentioned alignment method, there have been the following problems because the IF location sensor is inserted into an optical path of EUV light and blocks the EUV light at observation of the image of the EUV emission point.
(1) While the image of the EUV emission point is observed, it is impossible to supply EUV light to an exposure unit to perform exposure.
(2) When the EUV collector mirror is replaced, it is impossible to observe the image of the MTV emission point to perform automatic alignment.
(3) When the optics is out of alignment due to changes in thermal load of the EUV light source apparatus, it is impossible to detect a light profile (e.g., location and/or shape) of the image of the EUV emission point while the EUV light is supplied to the exposure unit.(4) When the optics is out of alignment due to changes in thermal load of the EUV light source apparatus, it is impossible to stabilize the image of the EUV emission point by feedback control while the EUV light is supplied to the exposure unit.